Without limiting the scope of the invention, the background is described in connection with plasma processing in the semiconductor industry, such as plasma etching or plasma deposition, as an example.
A plasma is an ionized gas which is used during the processing and manufacturing of semiconductor devices. For example, plasma is used to etch or remove material from the surface of semiconductor wafers. Plasma may also be used to deposit material onto the surface of semiconductor wafers. In another application, semiconductor wafers or substrates can be implanted with ions in a plasma in a process known as Plasma Immersion Ion Implantation (PIII). Thus, the use of plasma in the fabrication of semiconductor devices is wide spread in the microelectronics manufacturing industry.
Producing a plasma typically involves the use of a low pressure or vacuum chamber into which a processing gas is introduced. Once a plasma is generated within the processing vacuum chamber, the plasma surrounds the semiconductor wafer so that, for example, layers may be removed from the wafer surface or disposed thereon due to chemical reactions on the wafer surface caused by the plasma. The chamber, which is used to maintain the low pressure required for plasma processing, also serves as a structure for attachment of the plasma source. The plasma source or antenna, ionizes the processing gas within the chamber, thereby generating the plasma.
It has been found that while high density plasmas are desirable, conventional systems generate low density plasmas. This lead to the development of high density plasma sources such as inductively coupled plasma (ICP), ECR, and helicon sources. Inductively coupled plasma sources are generally in the form of a coil which couples the rf power to the processing gas through its rf magnetic fields. The magnetic fields penetrate a dielectric window into the processing chamber and induce an electric field that initiates and sustains the plasma by accelerating its free electrons. ICPs are attractive for the semiconductor industry because they can produce high densities at low pressures. Low pressure operation is required to control the anisotropy of the plasma etching to produce sub-half micron features, which allows higher device packing density and better device performance.
One type of inductively coupled plasma sources is a flat spiral coil such as that shown in U.S. Pat. No. 4,948,458. It generates a planar magnetic field that induces a circulating electric field, which greatly increases the electrons travel path in the plasma before they diffuse to the chamber walls. Moreover, as the electrons are closely confined to a plane that is parallel to the coil, transfer of kinetic energy to the ions in a direction perpendicular to the wafer surface is minimized, hence plasma damage to the wafer surface is reduced. As a result, the ion velocity component towards the wafer surface can be controlled by independently biasing the wafer electrode. This feature of ICPs allows independent control over the ion energy directed towards the wafer surface, while the plasma density can be increased independently by increasing the input power to the source. Conventional plasma processing systems, such as capacitively coupled plasma systems, do not have the ability to decouple the ion energy from the power deposition into the plasma.
Other coil configurations have been employed to generate inductively coupled plasmas. Solenoidal coils can be wrapped around a dielectric chamber to generate ICPs as described in U.S. Pat. No. 3,705,091. Two other configurations of ICP sources are described in U.S. Pat. No. 5,277,751 and No. 5,280,154 respectively. These patents describe a solenoidal coil with at least one flat side to provide a planar surface disposed against the dielectric window.
The uniformity of the plasma density affects the uniformity of the processing across the wafer surface and is another important aspect of plasma source design. A major problem caused by non-uniform plasmas is the uneven etching of transistor gate layers or the etching of the dielectric material around these layers. The deposition of various materials and removal of unwanted features by etching using to plasma is common throughout the industry. Due to uneven plasma etching over the wafer surface, it is general practice to employ an "over-etch" period in order to make sure that all unwanted features are cleared away.
It has been found that the use of a planar spiral coil may result in azimuthal process asymmetries and non-uniformities. For example, the uniformity of the plasma density and ion flux profiles to the wafer surface are greatly influenced by the transmission line properties and geometry of the spiral coil. Also, it has been observed that the placement of the spiral coil affects the etch rate profile. For example, a significant improvement in the etch rate profile uniformity has been found as the coil center was shifted from the wafer center, as well as a rotation in the etch rate profile non-uniformities when the coil was rotated by 180.degree. around its axis. Also, it has been found that a dielectric window with a thicker portion at the center improved the etch rate uniformity compared to a flat dielectric window as described in U.S. Pat. No. 5,226,967 and No. 5,368,710. The plasma density uniformity of a flat spiral coil can also be improved by placing magnetic dipoles around the processing chamber, which provides a surface magnetic field for confining the plasma as described in U.S. Pat. No. 5,304,279.
Unlike conventional plasma sources, ICP source geometry can easily be altered to improve the plasma uniformity across the wafer surface. An alternative configuration of an inductive coil, is a coil having a planar and a tubular portion as described in U.S. Pat. No. 5,309,063. It provided a more uniform plasma density across the chamber, compared to a planar spiral coil. The plasma ion flux uniformity to the wafer surface can be improved by contouring the spiral coil and the dielectric window as described in the U.S. Pat. No. 5,231,334. U.S. Pat. No. 5,346,578 describes an expanding spiral coil that has a hemispherical shape following the contour of a hemispherical shaped quartz bell jar, which serves as the processing chamber. This design achieved a good plasma ion current uniformity across a 200 mm wafer. A non-uniformly spaced spiral coil, described in U.S. Pat. No. 5,401,350, improved the plasma uniformity compared to an equally spaced spiral coil.
As the semiconductor industry shifts toward large area wafer processing, high density plasma sources that generate uniform plasmas over a large area are needed. ICP sources are good candidates to meet these challenges due to their construction simplicity and potential for scaling. For example, U.S. Pat. No. 5,261,962 describes a large area planar ICP antenna, formed by disposing straight conductor elements in the shape of a ladder. The antenna was used as a plasma source for a plasma enhanced chemical vapor deposition (PECVD) system. The deposited thin film was very uniform across large area substrates. U.S. Pat. No. 5,464,476 describes a large area substrate plasma source assembly. The source comprises a plurality of spiral coils placed adjacent to each other in a form of an array. Plasma processing of large workpieces was also described in U.S. Pat. No. 5,589,737. The plasma source is an ICP planar coil that has plural segments of equal length connected in parallel to an rf power source.
What is needed, therefore, is a source that generates a uniform high density plasma within a vacuum chamber that permits high processing rates and uniform deposition and etching of materials on semiconductor wafers. Also, what is needed is a source that can be scaled to process large area wafers, while maintaining high processing rates and uniformity.